Method and system for measuring a physical parameter of at least one layer of a multilayer article without damaging the article and sensor head for use therein

ABSTRACT

A method and system are provided to measure a physical parameter of a film such as film thickness or build, in both the wet and dry states of the film, on an article in a production environment without any contact or damage to the film or the rest of the article. The system includes two laser light sources, one pulsed and the other substantially continuous (i.e. long pulses), an interferometer, fiber optic cables, and appropriate optics including multiplexers and demultiplexers to: 1) image and transmit light from both sources to the film on the article; and 2) transmit the light from the continuous source reflected off the film surface back to the interferometer, and a computer containing a data acquisition card to record the data after signal processing and appropriate software to process the data. Light from both light sources are imaged on and over the same spot on the surface of the film. The pulsed light source is used to generate ultrasound in the film. The reflected light from the continuous source is detected by the interferometer to probe the ultrasonic vibrations of the film. The motion of the surface of the film at ultrasonic frequencies generates a Doppler shift in the frequency of the continuous light source. This modulation of the continuous light source frequency is monitored by the interferometer and is converted to a signal which is recorded and processed by the computer. Signal processing involves performing a Z-transform of the data to identify the resonance frequencies of ultrasonic motion in the film. These resonant frequencies, in conjunction with some physical properties of the material of the film are used to determine the film build of the layer.

This application claims the benefit of Provisional Application Ser. Nos.60/031,717 and 60/032,006, both filed Nov. 22, 1996.

TECHNICAL FIELD

The present invention relates to methods and systems for measuring aphysical parameter of at least one layer of a multilayer article andsensor head for use therein and, in particular, to laser ultrasonicmethods and systems for measuring a physical parameter such as thicknessof a solid or liquid film formed on an article without damaging the filmand a sensor head for use therein.

BACKGROUND ART

In any coating process of an article of manufacture such as anautomotive body, there is an optimum specification for the resultingfilm build, i.e., thickness of the resulting coating layer involvingacceptable performance, appearance and materials cost. The ability tomeasure this film build on-line in a production environment would bebeneficial to the manufacturer.

Often, any method for measuring the film build of a coating layer mustrequire that no contact with the film occur either to avoid degradingthe effectiveness or marring the appearance of the film.. This isespecially true for coatings while they are wet.

With manufacturing film build data, the bulk materials costs can becontrolled by applying the minimum amount of material to reach anacceptable film build. Other savings can also be realized, for examplemeasuring and improving the transfer efficiency of the coating processand correlating film build to the quality of the appearance of the finalcoated surface. An example process and production environment that wouldbenefit from the ability to measure film build on-line is the paintingof automobile bodies.

In automated painting operations, a prime concern is the reduction ofenvironmental impacts due to the evaporation of solvents. Means ofreducing the amount of solvent released into the atmosphere includeelectrostatic application of the paint and the use of waterborne paints.Electrostatic application increases the quantity of paint delivered tothe painted object, and thus reduces the total quantity of paintrequired due to the decrease in overspray. The use of waterborne paintsdramatically reduces the quantity of solvent utilized in the paintbecause water is used as a vehicle for paint delivery rather thansolvent. Environmental concerns may dictate the exclusive use ofwaterborne paints in the future.

In order to further reduce waste, thus reducing solvent emissions, andto improve the quality of the finished painted article, it may benecessary to monitor or sense various paint physical parameters such asthickness with precision to effect control.

Waterborne paints are electrically conductive and, therefore, must beisolated from the environment such that an electrostatic charge may beimparted to the flow of paint. This isolation must be at least 100kilovolts. Further, the painting environment is a hazardous environmentdue to the few remaining solvents in the paint. Therefore, any devicewhich meters or measures the physical parameters of paint must provideelectrostatic isolation and limit energies within the paintingenvironment to less than that required for ignition.

The painting process for automobiles involves applying several coatingsof different materials to an underlying metal or plastic substrate 10.As illustrated in FIG. 1, these coatings may include variousanticorrosion layers such as a phosphate layer 12, an E-coat layer 14,primer layer(s) 16, colored paint layers 18 (referred to as base coats),and a transparent protective and appearance improving material(s) calleda clearcoat 20. The ability to measure both total film build, i.e., thetotal thickness of all layers and/or the thickness of each individuallayer, in both the wet or dry states would be useful.

One non-contact method for measuring solid film thickness and/or otherphysical properties of the film is provided by ultrasound generation inthe film and subsequent ultrasound detection. However, this methodtypically locally damages or destroys the film.

For example, U.S. Pat. No. 4,659,224 discloses optical interferometricreception of ultrasound energy using a confocal Fabry-Perotinterferometer and the detection of the Doppler shift of the laser linefrequency as the method to detect the ultrasound.

U.S. Pat. No. 5,137,361 discloses optical detection of a surface motionof an object using a stabilized interferometric cavity. Theinterferometer cavity is stabilized by controlling the position of therear cavity mirror with a piezoelectric pusher.

U.S. Pat. No. 5,402,235 discloses imaging of ultrasonic-surface motionby optical multiplexing. Ultrasound is detected using an array ofdetectors and a "demodulator". The demodulator is typically aphotorefractive crystal within which a hologram of the laser beam, bothdirectly from the laser and reflected off the sample's surface, aresimultaneously written. The interference between sample laser beam andthe beam reflected off the sample surface takes place between the twoholographic images generated within the crystal.

U.S. Pat. No. 5,590,090 discloses an ultrasonic detector using verticalcavity surface emitting lasers. The method requires contact between thesample and the equipment.

U.S. Pat. No. 5,035,144 discloses frequency broadband measurement of thecharacteristics of acoustic waves. Propagating acoustic waves aredetected by two different broadband receivers at first and secondlocations separated by a distance L. The data analysis for this methodinvolves detailed comparisons between group and phase velocities of thedata using different amplifiers and narrow band filtering of the signal.

U.S. Pat. No. 5,604,592 discloses a laser ultrasonics-based materialanalysis system and method using matched filter processing. The systemuses a diode laser for detection. Generation and detection of ultrasoundis done at different points. The system relies on Time of Flight (TOF)detection which requires generation and detection at separate pointssince the basis of the measurement is the time it takes for theultrasonic energy to travel between the two points. The waveshape of thetime varying ultrasonic signal is acquired with a matched filter,processed and basically compared to a library of similar signals.

U.S. Pat. No. 5,615,675 discloses a method and system for 3-D acousticmicroscopy using short pulse excitation and a 3-D acoustic microscopefor use therein.

U.S. Pat. No. 5,305,239 discloses ultrasonic non-destructive evaluationof thin specimens. The method involves data analysis for thin specimens,where "thin" is defined as less than or equal to the wavelength of theinspecting acoustic wave. Analysis is demonstrated with a Fast FourierTransform (FFT). An important aspect of an FFT is that it can onlyproduce discrete frequency results determined by the number of pointstaken and data rate used.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and system foraccurately measuring a physical parameter such as thickness of at leastone layer of a multilayer article without damaging the article andsensor head for use therein wherein such measurement can be quickly andsafely performed in a factory environment so that the measurement can beused in a real-time, closed loop, feedback control system if desired.

Another object of the present invention is to provide a laser ultrasonicmethod and system for accurately measuring a physical parameter such asthickness of at least one layer of a multilayer article without damagingthe article and sensor head for use therein and which are relativelyinsensitive to article position and tilt and have a large depth offield.

Still another object of the present invention is to provide a laserultrasonic method and system for accurately measuring a physicalparameter such as thickness of a solid or liquid film formed on asubstrate layer without damaging the resulting multilayer article,wherein the system includes a plurality of sensor heads which canoperate within a potentially hazardous environment of a paint booth andwherein all light pulses and electrical signals are generated at acentral, secure location away from the hazardous environment.

Yet still another object of the present invention is to provide a laserultrasonic method and system for accurately measuring a physicalparameter such as thickness of a liquid film formed on an automotivepart without damaging the film, wherein the system includes a pluralityof sensor heads and wherein the automotive part moves relative to thesensor heads within a paint booth.

In carrying out the above objects and other objects and features of thepresent invention, a method is provided for measuring a physicalparameter of at least one layer of a multilayer article without damagingthe article. The method includes the steps of generating a first pulseof electromagnetic energy, transmitting the first pulse to a sensingstation, generating a second pulse of electromagnetic energy, andtransmitting the second pulse to the sensing station. The method alsoincludes the step of directing the first pulse at the sensing stationtoward a first spot on a surface of the article to generate anultrasonic wave in the article which, in turn, causes ultrasonic motionof the surface of the article without damaging the article. The methodfurther includes the step of directing the second pulse at the sensingstation toward a second spot on the surface the article whichsubstantially overlaps the first spot to obtain a reflected pulse ofelectromagnetic energy which is modulated based on the ultrasonic motionof the surface. Finally, the method includes the steps of transmittingthe reflected pulse from the sensing station, detecting the reflectedpulse after the step of transmitting the reflected pulse to obtain acorresponding ultrasonic electrical signal, and processing theultrasonic electrical signal to obtain a physical parameter signal whichrepresents the physical parameter of the at least one layer at the firstand second overlapping spots.

Preferably, the step of processing includes the step of initiallyprocessing the ultrasonic electrical signal to obtain a resonancefrequency signal based on resonance of the at least one layer and thenprocessing the resonance frequency signal to obtain the physicalparameter signal.

The multilayer article, such as a vehicle body, may include a metal,plastic or composite material substrate layer. Also, the physicalparameter may be thickness of a film layer or the substrate layer orsome other physical parameter such as percent solids, density orviscosity of the film layer which may be either a solid film layer orliquid film layer.

Preferably, the steps of generating the pulses and the step of detectingthe reflected pulse are performed at a location remote from the sensingstation.

Also, preferably, the first and second pulses of electromagnetic energysuch as light are substantially collinear at the first and secondoverlapping spots.

Also, preferably, the step of initially processing the ultrasonicelectrical signal includes the steps of digitizing the signal and thenapplying a Z-transform to the digitized ultrasonic electrical signal.

The sensing station may be located in a hazardous environment andwherein the steps of generating the first and second pulses, the step ofdetecting the reflected pulse and the step of processing the resultantsignal are performed outside of the hazardous environment.

In a production environment, the method further includes the step ofmoving the article relative to the first and second pulses during thesteps of directing the pulses. Also, the method further includes thestep of generating a position signal representative of position of thearticle relative to the first and second directed pulses at the sensingstation and wherein the physical property signal is processed with theposition signal to locate a defect in the article.

Still, preferably, the method includes the step of generating a controlsignal based on location of the defect in the article and eithercontrollably moving a surface coating mechanism to a surface coatingposition in response to the control signal to coat the surface of thearticle as a function of the location of the defect or using thiscontrol signal to adjust some aspect of the original coating equipment(e.g. paint flow controllers) or for some similar method of closed loopfeedback control.

Still further in carrying out the above objects and other objects of thepresent invention, a system is provided for measuring a physicalparameter of at least one layer of a multilayer article without damagingthe article. The system includes a generation laser for generating afirst pulse of electromagnetic energy, a first optical fiber fortransmitting the first pulse therethrough to a sensing station, adetection laser for generating a second pulse of electromagnetic energy,and a second optical fiber for transmitting the second pulsetherethrough to the sensing station. The system also includes a sensorhead coupled to the first and second optical fibers at the sensingstation. The sensor head has at least one optical component fordirecting the first pulse transmitted through the first optical fibertoward a first spot on the surface of the article to generate anultrasonic wave in the article which, in turn, causes ultrasonic motionof the surface of the article without damaging the article. The sensorhead also includes at least one optical component for directing thesecond pulse toward a second spot on the surface of the article whichsubstantially overlaps the first spot to obtain a reflected pulse ofelectromagnetic energy which is modulated based on the ultrasonic motionof the surface. The sensor head receives the reflected pulse. The systemfinally includes an optical detector coupled to the sensor head fordetecting the reflected pulse to obtain a corresponding ultrasonicelectrical signal, and a signal processor for processing the ultrasonicelectrical signal to obtain a physical parameter signal which representsthe physical parameter of the at least one layer at the first and secondoverlapping spots.

Preferably, the optical detector includes an optical interferometer suchas a confocal Fabry-Perot type interferometer.

Preferably, the lasers and the optical detector are located remote fromthe sensing station.

Still further in carrying out the above objects and other objects of thepresent invention, a sensor head adapted for use in the system isprovided. The sensor head includes a housing adapted to receive firstand second pulses of electromagnetic energy and a dichroic beam splittersupported within the housing for transmitting one of the pulses andreflecting the other pulse. The sensor head also includes a set ofoptical components also supported within the housing for (1) directingthe first pulse toward a first spot on a surface of the article, (2)directing the second pulse toward a second spot on the surface of thefilm which substantially overlaps the first spot to obtain a reflectedpulse of electromagnetic energy and, finally, (3) receiving thereflected pulse.

Preferably, the beam splitter and the set of optical components arearranged within the housing so that the first and second pulses at thefirst and second spots are substantially collinear.

Still further in carrying out the above objects and other objects of thepresent invention in a production environment, a method is provided formeasuring a physical parameter of at least one layer over an extendedarea of a multilayer article without damaging the article. The methodincludes the steps of generating a first pulse of electromagneticenergy, directing the first pulse into a first plurality of beams,transmitting the first plurality of beams to a plurality of separatelocations at a sensing station, generating a second pulse ofelectromagnetic energy, directing the second pulse into a secondplurality of beams, and transmitting the second plurality of beams tothe plurality of locations at the sensing station. The method alsoincludes the step of directing the first plurality of beams at thelocations at the sensing station toward a first plurality of spots on asurface of the article to generate ultrasonic waves in the articlewhich, in turn, causes ultrasonic motion of the surface of the articlewithout damaging the article. The method also includes the step ofdirecting the second plurality of beams at the sensing station toward asecond plurality of spots on the surface the article, each of the secondplurality of spots substantially overlapping one of the first pluralityof spots to obtain a reflected pulse of electromagnetic energy which ismodulated based on the ultrasonic motion of the surface at itsrespective first and second overlapping spots. Finally, the methodincludes the steps of transmitting the reflected pulses from the sensingstation, collecting the transmitted reflected pulses to obtain collectedpulses, detecting the collected pulses to obtain a plurality ofcorresponding ultrasonic electrical signals, and processing theplurality of ultrasonic electrical signals to obtain a plurality ofphysical parameter signals which represents the physical property of theat least one layer over the extended area of the article.

Preferably, the method includes the steps of moving the article relativeto the sensor station and generating a position signal representative ofposition of the article at the sensor station and wherein the physicalparameter signals are processed with the position signal to locate adefect in the extended area of the article.

Still, preferably, the method further includes the step of generating acontrol signal based on location of the defect and coating the surfaceof the article as a function of the control signal.

Yet still further in carrying out the above objects and other objects ofthe present invention, a system is provided for measuring a physicalparameter of at least one layer over an extended area of a multilayerarticle without damaging the article. The system includes a generationlaser for generating a first pulse of electromagnetic energy, a firstdevice for directing the first pulse into a first plurality of beams anda first plurality of optical fibers for transmitting the first pluralityof beams therethrough to a corresponding plurality of locations at asensing station. The system also includes a detection laser forgenerating a second pulse of electromagnetic energy, a second device fordirecting the second pulse into a second plurality of beams and a secondplurality of optical fibers for transmitting the second plurality ofbeams therethrough to the corresponding plurality of locations at thesensing station. The system further includes a plurality of sensorheads, each of the sensor heads is positioned at one of the locations atthe sensing station and is coupled to one of each of the first andsecond pluralities of optical fibers. Each of the sensor heads has atleast one optical component for directing one of the first plurality ofbeams toward one of a first plurality of spots on a surface of thearticle to generate an ultrasonic wave in the article which, in turn,causes ultrasonic motion of the surface of the article without damagingthe article. Each of the sensor heads also has at least one opticalcomponent for directing one of the second plurality of beams toward oneof a second plurality of spots on the surface of the article whichsubstantially overlaps its corresponding spot of the first plurality ofspots to obtain a reflected pulse of electromagnetic energy which ismodulated based on the ultrasonic motion of the surface at itsrespective first and second overlapping spots. Each of the sensor headsalso receives its reflected pulse. The system still further includes athird device coupled to each of the sensor heads for collecting thereflected pulses to obtain collected pulses, an optical detector coupledto the third device for detecting the collected pulses to obtain aplurality of corresponding ultrasonic electrical signals and a signalprocessor for processing the plurality of ultrasonic electrical signalsto obtain physical parameter signals which represents the physicalparameter of the at least one layer over the extended area on thearticle.

In one embodiment, the system includes a mechanism for moving thearticle relative to the sensor station and a sensor coupled to themechanism and the signal processor to generate a position signalrepresentative of position of the article relative to the sensor stationand wherein the physical parameter signal is processed with the positionsignal by the signal processor to locate a defect in the article at theextended area of the article.

Still, preferably, the system includes a controller coupled to thesignal processor for generating a control signal based on location ofthe defect in the article and a surface coating mechanism for coatingthe surface of the object as a function of the control signal.

In one embodiment, the mechanism includes a robot and the controller isa robot controller.

In one embodiment, the second plurality of optical fibers couple thesensor heads to the third device and in another embodiment a thirdplurality of optical fibers couple the sensor heads to the third device.

The method and system of the present invention provide numerousadvantages. For example, advantages that make the invention most suitedfor automotive paint film measurements are: 1) the measured parameter isthe frequency of the ultrasound and therefore the invention is notaffected by changes in the intensity of the signal (intensities aresensitive to a large number of factors (e.g., precise sensor-to-bodystandoff and orientation, small amounts of overspray build-up on optics)which do not affect this measurement); 2) the measurement is extremelyfast (˜5 μs) and thus the measurement is not sensitive to motion orvibrations in the body; 3) the impact of the above two points is thatone can use position insensitive sensors (minimal sensor/articlealignment is required); 4) the sensors can operate at a large standoff(i.e. 1 meter) from the article; 5) only minimal components (i.e.sensors) required in spray booth or hazardous area; 6) can operate inharsh environments, especially related to temperatures; 7) capable ofmeasuring all materials (composition, wet, dry); and 8) the equipmentcan be multiplexed (i.e., for example, 400 sensors) thus one set ofequipment can cover an entire assembly plant.

The above objects and other objects, features, and advantages of thepresent invention are readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating a typical automotive coatingprofile;

FIG. 2 is a schematic view of a simplified measuring system with atwo-fiber laser launch system of FIG. 5 and constructed in accordancewith the present invention;

FIG. 3 is a schematic view of a three-fiber sensor head constructed inaccordance with the present invention without its housing;

FIG. 4 is a schematic view of a two-fiber sensor head constructed inaccordance with the present invention;

FIG. 5 is a schematic view of a two-fiber laser launch systemconstructed in accordance with the present invention and utilized inFIG. 2;

FIG. 6 is a schematic view of a more complex system constructed inaccordance with the present invention with generation and detectionlasers and an interferometer of the system located in a laser roomremote from an automotive paint booth in which sensor heads of thesystem are located;

FIG. 7 is a schematic side elevational view of an automotive vehiclebody carried by a conveyor and a portion of a sensor arch having sensorheads constructed in accordance with the present invention;

FIG. 8 is a schematic top plan view of a plurality of sensor archespositioned at various locations within a paint booth;

FIG. 9 is a graph illustrating a laser ultrasonic signal obtained frommeasuring a wet green paint on a 1.0 mil E-Coat with a 1.6 mil primer;

FIG. 10 is a graph of the signal of FIG. 9 after transformed by adiscrete Fourier transform and which illustrates resonance frequenciesof the substrate and a paint film;

FIG. 11 is a set of graphs, each one of which is similar to the graph ofFIG. 10 and illustrating how paint peak resonant frequencies change withvarying wet paint thickness;

FIG. 12 is a graph illustrating how a calibration curve could begenerated from LU measured values of a wet white solvent base coat; and

FIG. 13 is a schematic block diagram flow chart illustrating aprocessing method and system which focuses on how raw ultrasonic signalsand/or data are analyzed.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to FIG. 2, there is illustrated a schematic view of asimplified system constructed in accordance with the present invention,generally indicated at 22. The system includes a generation laser 24, adetection laser 26, and an interferometer 28.

The generation laser 24 produces a very short pulse (˜10 ns) that isused to generate the ultrasound in a thin liquid or solid film such aspaint formed on the article 30. The generation laser 24 is preferably aNd:YAG laser with an added fixed OPO having a 1570 nm wavelength whichprovides <10 mJ/pulse at the measured article. The absorption of thislaser's light pulse energy causes a temperature rise in the film(certainly <5° C., probably less than 1° C.) which, in turn, produces anessentially instantaneously generated density gradient in the material.This density gradient produces ultrasound. (This type of ultrasoundgeneration is referred to as thermoelastic generation.) Thus, the shortlaser pulse is analogous to a quick hammer strike to a bell therebygenerating sound.

For wet films, a second method of ultrasound generation is also present.The energy in the pulsed laser 24 is very small, considerably too smallto affect the solid materials in the paint, however it will cause anextremely small portion of solvent (μL) on the film surface to bequickly evaporated. The movement of this solvent mass away from the filmproduces a force in the opposite direction (i.e., into the film) whichis very efficient at generating ultrasound. This ultrasound generationmethod is referred to as ablative generation, which typically damagesthe surface of the article inspected. The energies used for thisinvention will only ablate volatile liquids, not solids, thus no damageoccurs to the solid film material.

The detection laser 26 is preferably a Nd:YAG laser having a 1064 or 532nm wavelength, >2 MHz line-width and provides ≦500 mW of power at thearticle inspected. The laser 26 is a nominally continuous (wave, or CW)laser which is used to detect the minute motions (<1 nm) of the samplesurface due to the ultrasound. The laser 26 can be thought of asproducing a very long pulse especially when compared to the length ofthe pulse produced by the laser 24.

The laser 26 has a very narrow linewidth and thus it is possible torecord the ultrasonic surface motions by monitoring the laser frequencyas it is Doppler shifted by the ultrasonic motions. The resultingdetection laser light pulse reflected off the article 30 is coupled intothe interferometer 28 via an optical fiber 41. The interferometer 28strips away and records the modulations in this laser light pulse linefrequency. The frequency of the film resonance is directly related toits thickness, with a thicker film having a slower resonance, again likea bell.

The interferometer 28 is preferably a confocal Fabry-Perot typeinterferometer constructed generally in accordance with the teachings ofthe above-noted U.S. Pat. No. 5,137,361.

The system 22 also includes a first optical fiber 32 for transmittingthe first pulse generated by the generation laser 24 therethrough to asensor head, generally indicated at 34, which is shown positioned withina spray booth 36 in which the article 30 is located.

The system 22 also includes a second optical fiber 38 for transmitting asecond light pulse generated by the detection laser 26 therethroughafter striking a reflective element or prism 107 in a system 100 asillustrated in FIG. 5 which directs the second light pulse into theoptical fiber 38.

In general, the sensor head 34 directs the first light pulse appearingon the optical fiber 32 at a sensing station within the paint booth 36toward a first spot 42 along light signal path 44 on a surface of thefilm of the article 30 to generate an ultrasonic wave such as alongitudinal ultrasonic wave in the film which, in turn, causesultrasonic motion of the surface of the film without damaging the filmor the rest of the article 30.

In general, the sensor head 34 also directs the second pulse of lighttransmitted through the second optical fiber 38 at the sensing stationtoward a second spot 46 which is coincident and overlaps the first spot42 along a light signal path 48 to obtain a reflected pulse of lightwhich returns to the sensor head 34 along a path back to the sensorhead. The limits of such a path are indicated at 50 in FIG. 2. Thereflected pulse of light is modulated based on the ultrasonic motion ofthe surface of the film, as is well known in the art.

Referring to FIGS. 2 and 5, the sensor head 34 transmits the reflectedpulse of light from the sensing station to the optical fiber 38 and thenout of the fiber toward a lens 112 of FIG. 5. This light is coupled intothe fiber 41 by the lens 112 therethrough so that it is received by theinterferometer 28 along the optical fiber 41. The interferometer 28detects the reflected pulse of light to obtain a correspondingultrasonic electrical signal such as the one illustrated in FIG. 9.

A three-fiber system may be used instead of the two-fiber system of FIG.2 to eliminate the block 100. However, an extra fiber is needed asillustrated at 76 in FIG. 3.

As illustrated in the embodiment of FIG. 6, the interferometer 28 iscoupled to a computer 50 to process the ultrasonic electrical signalprovided by the interferometer 28 to obtain a physical property signalsuch as a thickness signal which represents the thickness of the film orsubstrate of the multilayer article 30. In general, the computer 50 isprogrammed to initially digitize the ultrasonic electrical signal toobtain a discrete-time signal and then, by applying a Z-transformthereto, to obtain the resonance frequencies in the ultrasonic signal.The resonant frequencies obtained from the ultrasonic signal in FIG. 9are illustrated in FIG. 10. These frequencies in FIG. 10 were obtainedby a Fourier transform as opposed to a Z-transform for this illustrativeexample due only to the ease of graphing the output of a Fouriertransform. The actual method to extract the resonant frequencies is witha Z-transform and then the computer 50 processes the resonancefrequencies to obtain the thickness signal as described in greaterdetail hereinbelow.

Referring now to FIG. 3, there is illustrated one embodiment of a sensorhead, generally indicated at 52 and constructed in accordance with thepresent invention. The sensor head 52 typically includes a supporthousing (which housing is not shown in FIG. 3 for simplicity). Thesensor head 52 receives the first pulse of light from a coupler 54formed on one end of the fiber 32 to provide an optical signal 56 whichis transmitted through a dichroic beam splitter 58.

The sensor head 52 also supports a coupler 60 which couples the opticalfiber 38 to the sensor head 52 to transmit a laser light signal 62 tothe beam splitter 58 which reflects the second pulse of light. A lens 64focuses the first and second pulses of light to a prism or mirror 66which, in turn, reflects the first and second pulses of light alonglight signal path 68 to the spots 42 and 46 on the article 30. Thus, thefirst and second pulses of light are directed toward the articlecollinearly.

The resulting reflected pulse of light travels along any path within thelimits set by path 70 to a lens 72 which, in turn, focuses the reflectedpulse of light to a coupler 74. The coupler 74 is coupled to acollection fiber 76 which returns the reflected pulse of light to theinterferometer 28 thereby eliminating the necessity of having the system100.

Referring now to FIG. 4, there is illustrated a second embodiment of asensor head, generally indicated at 78. The sensor head 78 may beutilized in the system 22 since optical fiber 38 serves not only as adelivery fiber but also as a collection fiber.

The sensor head 78 includes a housing 80 for housing optical componentsof the sensor head therein. A typical standoff between the housing 80and the article 30 is 150 mm but can be widely varied. The sensor head78 includes a dichroic beam splitter 82 supported within the housing 80to transmit the first pulse of light generated by the generation laser24 appearing along path 84 as emitted from a coupler 86 which couplesthe optical fiber 32 to the housing 80 of the sensor head 78.

The beam splitter 82 also reflects the second pulse of light from theoptical fiber 38 through a coupler 88 along a path 90 so that theresulting first and second pulses of light are collinear within thesensor head 78 along a path 92 within the housing 80.

The sensor head 78 also includes a lens 94 for imaging the first andsecond pulses of light along a path 96 to the spots 42 and 46,respectively, on the film layer of the article 30. In turn, a reflectedpulse of light travels along any path within the limits set by path 96through the lens 94, travels along any path within the limits set bypath 92, is reflected by the beam splitter 82 to travel along any pathwithin the limits set by the path 90 back to the coupler 88 and backthrough the optical fiber 38 to the optical system 100 and to theinterferometer 28 through the fiber 41.

Referring again to FIG. 5, there is illustrated a schematic view of atwo-fiber laser launch system for the second pulse of light source. Ingeneral, the laser launch system 100 illustrates in detail how thesecond pulse of light is transmitted to the optical fiber 38 as well ashow the reflected pulse of light is transmitted from the optical fiber38 to the optical fiber 41 and then to the interferometer 28.

The system 100 includes a shutter 102 which allows a portion of thesecond pulse of light to pass therethrough along a path 104 to afocusing lens 106 which focuses the second pulse of light to a mirror orprism 107 which, in turn, directs the second pulse of light to a coupler108 which couples the second pulse of light to the optical fiber 38.

The reflected pulse of light is transmitted by the optical fiber 38 tothe coupler 108 and, after being emitted therefrom, travels along thepath 110 to the condenser lens 112 for focusing to the optical fiber 41by means of a coupler 114.

Referring now to FIG. 6, there is illustrated a more complex system,generally indicated at 120, also constructed in accordance with thepresent invention. The generation laser 24, the detection laser 26 andthe interferometer 28 of the system are located in a laser room 122which is remote from a paint booth 124 in which a plurality of sensorheads, such as the sensor head 34, are located at a plurality of sensorstations 130.

The system 120 includes multiplexing and demultiplexing elements 126 formultiplexing or directing the first and second pulses of light generatedby the generation laser 24 and the detection laser 26 into first andsecond pluralities of beams of light and for demultiplexing orcollecting the reflected pulses of light for detection by theinterferometer 28. Optical fiber bundles 128 extend between the room 122and local controllers 129 located near the paint booth 124 adjacenttheir respective sensor stations 130. The fiber bundles 128 providepaths for the first and second plurality of beams of coherent light andthe reflected pulses of light between the two areas. Each localcontroller 129 includes a computer for controlling multiplexing elements(not shown) for multiplexing or directing the first and second pulses oflight generated by the generation laser 24 and the detection laser 26and demultiplexers (not shown) for demultiplexing or collecting thereflected light pulses for ultimate use by the interferometer 28.

In this manner, only a few number of optical fibers need be connectedbetween the laser room 122 and the local controllers 129 which typicallywill be widely separated. A larger number of optical fibers 131 whichextend from the local controllers 129 to the plurality of sensor heads,such as sensor head 34, are of a much shorter length, thus reducing thetotal amount of optical fiber required. In systems with a few number ofsensor heads, the local controller multiplexer can be omitted.

In the embodiment of FIG. 6, multiplexing is utilized to take multiplemeasurements with the above components. Multiplexing both light sources24 and 26 and the reflected light pulses from the car body surfacescouple the light beams into the fiber optic cables 128. The lightsources 24 and 26 and the interferometer 28 are located remotely fromautomotive body assemblies 136.

With the use of acousto-optic (AO) cells and/or reflective optics ongalvanometers and/or translating fiber optical switches (or any form ofoptical switches), pulses from the light sources 24 and 26 can bedirected into a specified fiber of a large set of fibers. In conjunctionwith directing pulses from the light sources 24 and 26 into differentfibers, the output from the fiber which collects the reflected lightfrom the film surface is directed to the interferometer 28 for recordingand processing.

The sensor heads 34 are positioned to make measurements at the desiredmultiple remote positions at the sensor stations 130. The second lightsource 26 and the light reflected from the film of the article areeither carried on separate fibers or combined into a single fiber with abeam splitter or similar optic.

In this manner, the film build measurement of many different positionswith the same light sources and interferometer is achieved.

As illustrated in FIGS. 6 and 8, the article may be an automotiveassembly 136 of metal parts which moves relative to arches 142 of sensorheads 34 of the system. In such a system, position signals generated byan encoder and limit switches (not specifically shown but located withinthe spray booth) coupled to the moving assembly 136 through a carriage134 and connected to the local controllers 129 are processed togetherwith the measurements at the computer 50 to locate those areas of thefilm which are either too thin or too thick.

The method and system may be utilized with a robot controller 138 whichcontrols a robot 140 to spray coat a surface of the assembly 136 at thelocation of the thin film. Another method would utilize the thicknesssignals as a closed loop feedback control to adjust some parameter(s)(e.g. paint flow control) of the process.

In the example illustrated in FIG. 6, each optical fiber bundle 128preferably includes two optical fibers to service each local controller129. Each optical fiber bundle 131 preferably includes eighteen opticalfibers to service the nine total sensor heads 34 positioned at eachsensing station 130 within the paint booth 124.

The computer 50 operates as a data collection and analysis device. Aseparate computer system controller 141 serves as a controller forcontrolling operation of the generation laser 24, the detection laser26, the interferometer 28, and the multiplexing and demultiplexingelements 126. The system controller 141 includes input/output circuitsnot only for control purposes but also to allow the separate controller141 to communicate with either the limit switches and the encoder withinthe spray booth directly or, preferably, with other computers of thelocal controllers 129 which monitor these signals.

The encoders within the spray booth are coupled to an assembly line 133which, in turn, moves carriages 134 which support the automotive bodyassemblies 136 as illustrated in FIG. 7.

The sensors shown in FIG. 7 may be fixed or contain rotation componentsto allow the sensors to remain pointed approximately at the normals tothe article.

The input/output circuits in the local controllers 129 allow itscomputer to communicate with the encoder(s) which generates a positionsignal representative of the position of each carriage 134. Preferably,each encoder is an optical incremental encoder mounted to a return wheelabout which a chain drive of the assembly line 133 moves. The localcontrollers 129 know when to look at the position signals provided bytheir encoders through the use of their limit switches which generatesignal inputs to the input/output circuits of the separate computer inthe laser room when the carriages 134 holding the assemblies 136 reachpredetermined positions within their sensor stations 130.

The data analysis computer 50 is coupled to a computer 137 whichreceives data from the data analysis computer 50 so that the computer137 can locate a surface defect in the film on the body assembly 136.The second computer 137 communicates with the computer 50 over a datalink 139 and which is located outside the room 122 to report thethickness which represents the thickness of the film or the substrate asdescribed hereinbelow. There will be one networked data link between allthe computers wherever they are located. To communicate among thecomputers one may install a LAN.

A signal is sent by the system controller 141 through its input/outputcircuits to the robot controller 138 which, in turn, generates a controlsignal based on location of the surface defect in the film on theassembly 136. The control signal from the robot controller 138 is usedby the robot 140 which is movable to a surface coating position tofurther coat the surface of the layer of the assembly 136.

Referring now to FIG. 7, there is illustrated a sensor head arch,generally indicated at 142, which includes a plurality of sensor heads34 mounted on vertically and horizontally extending beams 144 along thepath of the automotive assemblies 136 so that measurement readings canbe taken at various spots such as spots 146 along the car body assembly136. These sensors may be fixed as shown or contain rotation componentsto keep the sensor, either individually or in groups, pointedapproximately normal to the article at each inspection point 146.

Referring now to FIG. 8, there is illustrated a top plan view of aplurality of sensor arches such as the sensor arch 142 through whichautomotive car body assemblies 136 travel within a paint spray booth148. The laser room 122 contains the generation laser 24, the detectionlaser 26, the interferometer 28, the multiplexing/demultiplexingelements 126, the computer 50, and the separate computerized systemcontroller 141. Optical fiber bundles 128 transmit multiplexed laserlight signals therethrough to either various local controllers 129 andthen on to the sensor stations 130 in the booth 148 or directly to thesensor stations 130 themselves.

Referring now to FIG. 11, there is illustrated a set of graphs, each oneof which is similar to the graph of FIG. 10 and which illustrate how theresonant paint frequencies change with varying wet paint thickness.

FIG. 12 is a graph illustrating how a curve has been generated from LUmeasured values of wet white solvent base coat which calibration curvemay be stored within the computer 50 for data analysis. Preferably, thedata analysis computer 50 passes the resonant frequency signal of FIG.10, along with previous measurements made on the film or substrate andhence stored in the computer 50, into a model also stored in thecomputer 50 to determine film build of the film or the substrate on theassembly 136 as described in greater detail hereinbelow.

Summary of Data Analysis In brief, there are three main components tothe data analysis of the method and system of the present invention:signal processing; signal modeling; and wet-film to cured-filmprediction.

For the particular case of laser-based ultrasonics, the method andsystem operate in accordance with the block diagram flow chart of FIG.13.

The flow chart of FIG. 13 is an example of the process involved in a wetpaint measurement. Briefly, the programmed computers 50 and 137transform the raw ultrasound signals or data at block 182 from theinterferometer 28 into amplitudes, frequencies, and decay rates afterprocessing at block 184. The frequencies measured are used in a model(physical model or calibration model) to yield a thickness measurementas indicated at block 186.

In the case of automotive coatings, one assumes the measurement ofmultiple plane-layered systems. The layers can be described as:

Substrate layer: usually steel, aluminum, plastic, or composite

Static film layer: usually cured paint, primer, e-coat, clearcoat,phosphate

Dynamic film layer: usually wet or dehydrated paint, primer, clearcoat,e-coat, etc. Thickness and other physical properties, changing withtime.

The method and system can be utilized to measure static and dynamicfilms There are several methods by which one can convert frequency tothickness. One can use empirical calibration to convert frequency todried film thickness. The main disadvantages to calibrating dynamicfilms are the inclusion of extra variables. Some of these would be:

elapsed time of measurement from time of spray

temperature

humidity

preparation of paint

atomization pressure

fan patterning

paint composition

These effects may be impossible to quantify accurately.

In essence, a film loses base material via evaporation. The method andsystem of the present invention measure the amount of base and theamount of solids at the instant of measurement. The method and systemhave no need to include any of the aforementioned environmentalvariables that affect the wet film.

Signal Processing of Block 184

The invention models ultrasound as the summation of M decayingsinusoids. The modeling process is: find frequencies and decay ratesfrom raw data; and perform harmonic analysis of raw data using theseresults.

Find Frequencies and Decay Rates From Raw Data

One measures frequencies and decay rates from raw ultrasound data orsignals by pole-zero modeling. Specifically, the ultrasound signals onereceives can be modeled as exponentially decaying sinusoids. Thisproblem was first solved by Prony in 1795, and the basic mathematics aresubstantially identical to what one uses today. Only the computation ofthe math is modernized. Basically, a decaying complex sinusoid forms apole in the Z-transform domain. However, only frequency and decay rateinformation are contained in that pole as discussed by Oppenheim andSchafer in their book "Discrete-Time Signal Processing", Prentice Hall,1989:

    y(n)=Ae.sup.(-α+j2πf)n u[n]⃡A(1-e.sup.(-α+j2πf) z.sup.-1).sup.-1

where A is the amplitude of the signal, α is the decay rate, f isfrequency, and n is the discrete-time index.

By converting the problem of finding poles to a problem of findingzeros, the algorithm becomes:

Least-squares fit a linear prediction filter to the poles from raw data,i.e. transform signal to z-domain.

Find the zeros of the above filter in the z-domain.

Convert zeros to frequencies and decay rates.

Where:

L=size (order) of filter;

M=number of decaying sinusoidal signals in the raw data-in general M<L;and

N=number of raw data points to transform.

a. Least Squares Fit A Linear Prediction Filter to the Poles From RawData

From Prony's method, one solves the following equations to find thefilter: ##EQU1## A b=h h=[-y*(0),-y*(1), . . . , -y*(N-L-1)]^(T)

L=size of filter

N=number of raw data points to transform

y* is the complex conjugate of the data.

The above equation is very difficult to solve not only because ofnoisiness in y, but also because L>M. Essentially, two (or more) rows inthe matrix can become equal and cause a zero to appear in adenominator--one says that the problem is poorly conditioned orill-conditioned.

In laser ultrasonics of the present invention, the SNR rangesapproximately from 0 to 40 dB, i.e. low to moderate SNR. Moreimportantly, the number of sinusoids dynamically changes with the filmlayer system being measured. Therefore, one needs a robust method toextract the filter from the noisy data.

The SVD, or Singular Value Decomposition method, works quite well on theabove equations given the data laser ultrasound produces. The SVDmethod, as applied to the decaying sinusoid problem, was first describedby Kumaresan and Tufts in 1983 and is often called the KT method.

Basically, one solves the filter equation using SVD. With SVD, one canuse a filter whose order is larger than the number of signals oneexpects to receive. Increasing the filter size causes A to becomesingular. An intermediate solution then truncates the final solution toapproximate the number of signals. This improves the accuracy of the fitto the pole locations.

Presently, however, there is no need to truncate the SVD to obtaindesired results for frequency precision. Truncation often destroyed thedetectability of important signals (like low amplitude metal signals).Instead, "important" or desired signals are identified by amplitudes andknowledge of the film layer system.

b. Find The Poles of the Filter

The zeros of B(x) contain M signal zeros and L-M extraneous (or noise)zeros. The frequency/decay-rate information is extracted by rooting thefilter polynomial B(x). For example, consider a one-signal system:

    B(x)=1-e.sup.(-α+j2πf) x=0⃡x=e.sup.(-α+j2πf)

B(z) is changed to B(x) to emphasize that the problem is changed fromfinding poles to finding zeros. Secondly, the signal zeros will havepositive decay rates (for exponentially damped signals) while the noisezeros will have negative decay rates. This is because noise level istheoretically constant (i.e. non-decaying). Finally, for zero decaysignals, noise can "push" the measured decay rate to a negative value.

Because of the fairly large order of polynomial, one needs a very robustmethod of extracting the poles from the filter. Iterative methods areslow and generally fail for large polynomials. An eigenvalue method isused to perform this calculation.

To find eigenvalues of a matrix, one performs the operation:

    det[W-xI]=0

where det[] is the determinant of the matrix W-xI. The problem is set upusing the filter coefficients from B(x) in a companion matrix: ##EQU2##

Then, one can use basically the same theory (and mathematics) involvedin SVD to find the eigenvalues of the new matrix. Specifically, a QRmethod is used to estimate the eigenvalues. Specifics of how to computethe QR decomposition are well known.

c. Convert From Roots To Frequencies And Decay Rates

The signal zeros are sorted from the noise zeros by ignoring all |z|<cwhere for now c=1.000000. This value may be changed to a number like0.980000 because sometimes the metal peaks' signal zeros are pushedinside |z|<1 by noise (these generally lie at about 0.9995 or 0.995).

To determine frequency f and decay α, one uses the following:

    f=a tan(z.sub.i /zr)/2π

    α=-log(|z|)

Perform Harmnonic Analysis of Block 184

In order to obtain Amplitudes, one simply performs a harmonic dataanalysis using the frequencies and decay rates obtained above.Essentially, one performs a general linear least squares fit to the rawdata: ##EQU3##

    Ka=y

    a-[a.sub.1, a.sub.2, . . . , a.sub.L ].sup.T

    y-[y(0),y(1), . . . ,y(N-1)].sup.T.

These equations are not ill-conditioned and are thus easy to solve bygeneral linear least squares. Specifically, a simple Gauss-Jordantechnique is used to invert K. This technique is also very well knownand specifics of computation will not be discussed further.

Signal Modeling of Block 186

Modeling of the propagation of ultrasound in the layers is important toany non-calibrating measurement, as described herein. The system ismodeled as a compressional wave traversing through plane layers.

Given enough information (i.e. from block 188) and enough signal (i.e.from block 184), one can find (true) wet thickness. Additionally, onecan find speed of sound, density, and possibly viscosity andattenuation. The acoustic impedance of a material is the product ofspeed of sound and density:

    Z=ρc

Acoustic impedance controls how a wave is transmitted/reflected at aboundary (i.e. free, rigid, or other reflection) and how the "paint"frequencies interact with "metal" frequencies (and vice versa).Thickness and speed of sound determine how quickly the wave propagatesthrough a layer. Attenuation describes how much energy the wave loses(to heat or other) as it propagates through the layer. Given the modelof block 186, as well as good data from block 184, one can solve for anyof speed of sound, thickness, density, and/or attenuation usingoptimization techniques.

The resonance frequency model of block 186 is used to obtain thickness,speed of sound, and density information. Consider a lossless singlelayer film on a lossless steel substrate. The model for frequencyresonance for this particular system is: ##EQU4## where f is anyresonant frequency (paint or metal), d is thickness, c is speed ofsound, subscript p refers to paint layer qualities, and subscript srefers to steel substrate qualities.

This equation can be solved numerically. Specifically, one minimizesQ(d,c,ρ,f) for desired variables/parameters/data. However, there aremany ways to perform the minimization. For example, since one knows thevarious properties of the steel, one minimizes the following: ##EQU5##where i=1, 2, . . . number of measured resonances. The ; symbolrepresents that z and d/c are parameters to be optimized while f_(i) isinput data. The minimization is performed using the well known simplexmethod, and will not be discussed in detail here.

Wet Prediction

The process to predict a cured film thickness from a wet film thicknessis a two-step process:

Perform wet to uncured prediction of block 192;

estimate wet to uncured prediction factor from data of block 190;

calculate uncured thickness; and

Perform uncured to baked or cured prediction of block 194.

Perform Wet to Uncured Prediction (Block 192) Estimate Wet to UncuredPrediction Factor Via Laser-Based Ultrasound (Block 190)

Solving for the quantities such as z and d/c in the wet layer isimportant. For the particular case of ultrasonic measurement, onemeasures impedance and one-way transit time of the wet layer--call themZ_(wet) (d/c)_(wet), respectively. Density and speed of sound vary withparticulate size and particulate concentration, as well as solventproperties. If one can measure impedance, one can then estimate percentsolids or particulate concentration--call percent solids R_(wet) (letR_(wet) be a pure ratio less than 1 and not a percent). Symbolically,one measures R_(wet) (or (Rv)_(wet)) from Z_(wet) as:

    R.sub.wet V.sub.wet= (Rv).sub.wet =f(i Z.sub.wet).

One can also use this data to measure d_(wet).

The functionality of f() is either derived or measured, depending onapplication, knowledge available, etc. Note that it is very possible tomeasure f() using results from other techniques, say Time ResolvedInfrared radiometry (TRIR), as well as using this same process for wetprediction. See the next section for details.

Calculate Uncured Thickness (Block 192)

Once R_(wet) and d_(wet) (or even (Rv)_(wet) and (d/v)_(wet) for thespecial case of this laser ultrasonic application) are estimated, one isready to perform wet prediction. The following wet film to dry filmthickness prediction conversion is independent of measurement method.

Percent solids or particulate concentration tells what the wet layerwill reduce to if all of the solvent is removed. The predictivethickness of the uncured but base-free paint is called d_(uncured).Mathematically, this is expressed as:

    d.sub.uncured =R.sub.wet d.sub.wet

or

    d.sub.uncured =(Rv).sub.wet (d/v).sub.wet.

As an example, if one measures 4 mils thick wet water-based (=d_(wet)=4.00 mil) at 75% solids (=R_(wet) =0.75), that means the wet layer willreduce to 3 mils (=d_(uncured) =R_(wet) d_(wet) =0.75×4.00 mil=3.00mil).

Perform Uncured to Baked (Le. Cured) Prediction (Block 194)

One must also know how the uncured wet layer chemically reacts to become"paint". However, there is basically no way to predict how wet paintchemically reacts via ultrasound, thermal, image interferometry,microwave, millimeter wave, capacitance, etc., unless the reactionitself is monitored.

One measures the prediction in a laboratory or through knowledge of thechemical reaction. One method used with the present invention is tomeasure the prediction using density, as indicated at block 196. Thedensity of cured paint will be measured--call it ρ_(cured). The densityof uncured (and solvent- or water-free) paint will be measured--call itρ_(uncured). Because the paint has been applied to a surface, the areaof the uncured and cured paint is constant. Therefore, the uncured tocured paint prediction is:

    d.sub.cured ρ.sub.cured =d.sub.uncured ρ.sub.uncured.

One can then predict the final, baked or cured thickness, as indicatedat block 198.

It is possible that the above method can be used to measure wet glue todry glue thickness, as well as other properties of the glue. Moltencoatings, certain composites, etc. would also be measurable with themethod and system of the present invention.

Wet Film Prediction and Measurement

Determination of the Functionality of R_(wet)

There are several ways to determine the functionality of R_(wet). Firstis to use theory of the basic measurement technique. For example, onecould incorporate particle size, particle composition, particleconcentration, base composition, particle-base interaction, frequency,etc. into a physical model for f().

Another method to determine the functionality of R_(wet) =f() for agiven paint is calibration. As an example, consider measurement ofR_(wet) =f(c_(wet)) for black solvent-based paint. One can prepare manysamples of paint with various R_(wet) and measure the correspondingc_(wet). Then a curve can be fit in order to determine R_(wet)=f(c_(wet)).

While the best mode for carrying out the invention has been described indetail, those familiar with the art to which this invention relates willrecognize various alternative designs and embodiments for practicing theinvention as defined by the following claims.

What is claimed is:
 1. A method for measuring a physical parameter of atleast one layer of a multilayer article without damaging the article,the method comprising the steps of:generating a first pulse ofelectromagnetic energy; transmitting the first pulse to a sensingstation; generating a second pulse of electromagnetic energy;transmitting the second pulse to the sensing station; directing thefirst pulse at the sensing station toward a first spot on a surface ofthe article to generate an ultrasonic wave in the article which, inturn, causes ultrasonic motion of the surface of the article withoutdamaging the article; directing the second pulse at the sensing stationtoward a second spot on the surface of the article which substantiallyoverlaps the first spot to obtain a reflected pulse of electromagneticenergy which is modulated based on the ultrasonic motion of the surface;transmitting the reflected pulse from the sensing station; detecting thereflected pulse after the step of transmitting the reflected pulse toobtain a corresponding ultrasonic electrical signal; and processing theultrasonic electrical signal to obtain a physical parameter signal whichrepresents the physical parameter of the at least one layer at the firstand second substantially overlapping spots.
 2. The method as claimed inclaim 1 wherein the step of processing includes the step of initiallyprocessing the ultrasonic electrical signal to obtain at least oneresonance frequency signal based on resonance of the at least one layerand then processing the at least one resonance frequency signal toobtain the physical parameter signal.
 3. The method as claimed in claim1 wherein the article includes a metal substrate layer.
 4. The method asclaimed in claim 1 wherein the article includes a plastic substratelayer.
 5. The method as claimed in claim 1 wherein the article is avehicle body.
 6. The method as claimed in claim 1 wherein the at leastone layer is a solid film.
 7. The method as claimed in claim 1 whereinthe at least one layer is a liquid film.
 8. The method as claimed inclaim 1 wherein the physical parameter is thickness of the at least onelayer.
 9. The method as claimed in claim 1 wherein the ultrasonic waveis a longitudinal ultrasonic wave.
 10. The method as claimed in claim 1wherein the steps of generating the first and second pulses and the stepof detecting the reflected pulse are performed at a location remote fromthe sensing station.
 11. The method as claimed in claim 1 wherein thefirst and second pulses are substantially collinear at the first andsecond overlapping spots.
 12. The method as claimed in claim 2 whereinthe step of initially processing the ultrasonic electrical signalincludes the steps of digitizing the ultrasonic electrical signal toobtain a digitized ultrasonic electrical signal and applying aZ-transform to the digitized ultrasonic electrical signal.
 13. Themethod as claimed in claim 1 wherein the sensing station is located in ahazardous environment and wherein the steps of generating the first andsecond pulses and the steps of detecting the reflected pulse andprocessing the ultrasonic electrical signal are performed outside of thehazardous environment.
 14. The method as claimed in claim 1 furthercomprising the step of moving the article at the sensing stationrelative to the first and second pulses during the steps of directingthe first and second pulses.
 15. The method as claimed in claim 14further comprising the steps of generating a position signalrepresentative of the position of the article relative to the sensorstation and wherein the physical parameter signal is processed with theposition signal to locate a defect in the article.
 16. A system formeasuring a physical parameter of at least one layer of a multilayerarticle without damaging the article, the system comprising:a generationlaser for generating a first pulse of electromagnetic energy; a firstoptical fiber for transmitting the first pulse therethrough to a sensingstation; a detection laser for generating a second pulse ofelectromagnetic energy; a second optical fiber for transmitting thesecond pulse therethrough to the sensing station; a sensor head coupledto the first and second optical fibers at the sensing station and havingat least one optical component for directing the first pulse transmittedthrough the first optical fiber toward a first spot on a surface of thearticle to generate an ultrasonic wave in the article which, in turn,causes ultrasonic motion of the surface of the article without damagingthe article, the sensor head also having at least one optical componentfor directing the second pulse toward a second spot on the surface ofthe article which substantially overlaps the first spot to obtain areflected pulse of electromagnetic energy which is modulated based onthe ultrasonic motion of the surface, the sensor head receiving thereflected pulse; an optical detector coupled to the sensor head fordetecting the reflected pulse to obtain a corresponding ultrasonicelectrical signal; and a signal processor for processing the ultrasonicelectrical signal to obtain a physical parameter signal which representsthe physical parameter of the at least one layer at the first and secondoverlapping spots.
 17. The system as claimed in claim 16 wherein thesignal processor processes the ultrasonic electrical signal to obtain aresonance frequency signal based on resonance of the at least one layerand processes the resonance frequency signal to obtain the physicalparameter signal.
 18. The system as claimed in claim 16 wherein theoptical detector includes an optical interferometer.
 19. The system asclaimed in claim 18 wherein the optical interferometer is a confocalFabry-Perot type interferometer.
 20. The system as claimed in claim 16wherein the article includes a metal substrate layer.
 21. The system asclaimed in claim 16 wherein the article includes a plastic substratelayer.
 22. The system as claimed in claim 16 wherein the article is avehicle body.
 23. The system as claimed in claim 16 wherein the physicalparameter is thickness of the at least one layer.
 24. The system asclaimed in claim 16 wherein the ultrasonic wave is a longitudinalultrasonic wave.
 25. The system as claimed in claim 16 wherein thelasers and the optical detector are located remote from the sensingstation.
 26. The system as claimed in claim 16 wherein the at least onelayer is a solid film.
 27. The system as claimed in claim 16 wherein theat least one layer is a liquid film.
 28. The system as claimed in claim16 wherein the first and second pulses are substantially collinear atthe first and second overlapping spots.
 29. The system as claimed inclaim 16 wherein the signal processor initially processes the ultrasonicelectrical signal by digitizing the ultrasonic electrical signal toobtain a digitized ultrasonic electrical signal and then by applying aZ-transform to the digitized ultrasonic electrical signal.
 30. Thesystem as claimed in claim 16 wherein the sensing station is located ina hazardous or remote environment and wherein the lasers, opticaldetector and the signal processor are located outside of the hazardousor remote environment.
 31. The method as claimed in claim 16 furthercomprising means for moving the article at the sensing station relativeto the first and second pulses.
 32. The system as claimed in claim 31further comprising a sensor coupled to the means for moving and thesignal processor to generate a position signal representative ofposition of the article relative to the sensor station and wherein thephysical parameter signal is processed with the position signal by thesignal processor to locate a defect in the article.
 33. The system asclaimed in claim 32 further comprising a controller coupled to thesignal processor for generating a control signal based on location ofthe defect in the article.
 34. The system as claimed in claim 33 furthercomprising a surface coating mechanism for coating the surface of thearticle as a function of the control signal.
 35. The system as claimedin claim 34 wherein the mechanism includes a robot and wherein thecontroller is a robot controller.
 36. A sensor head adapted for use in asystem for measuring a physical parameter of at least one layer of amultilayer article without damaging the article, the sensor headcomprising:a housing adapted to receive first and second pulse ofelectromagnetic energy; a dichroic beam splitter supported within thehousing for transmitting one of the pulses and reflecting the otherpulse; and a set of optical components supported within the housing for:(1) directing the first pulse toward a first spot on a surface of thearticle; (2) directing the second pulse toward a second spot on thesurface of the article which substantially overlaps the first spot toobtain a reflected pulse of electromagnetic energy; and (3) receivingthe reflected pulse.
 37. The sensor head as claimed in claim 36 whereinthe beam splitter and the set of optical components are supported withinthe housing so that the first and second pulses are substantiallycollinear at the first and second overlapping spots.
 38. A method formeasuring a physical parameter of at least one layer over an extendedarea of a multilayer article without damaging the article, the methodcomprising the steps of:generating a first pulse of electromagneticenergy; directing the first pulse into a first plurality of beams;transmitting the first plurality of beams to a plurality of separatelocations at a sensing station; generating a second pulse ofelectromagnetic energy; directing the second pulse into a secondplurality of beams; transmitting the second plurality of beams to theplurality of locations at the sensing station; directing the firstplurality of beams at the locations at the sensing station toward afirst plurality of spots on a surface of the article to generateultrasonic waves in the article which, in turn, causes ultrasonic motionof the surface of the article without damaging the article; directingthe second plurality of beams at the locations at the sensing stationtoward a second plurality of spots on the surface the article, each ofthe second plurality of spots substantially overlapping one of the firstplurality of spots to obtain a reflected pulse of electromagnetic energywhich is modulated based on the ultrasonic motion of the surface at itsrespective first and second overlapping spots; transmitting thereflected pulses from the sensing station; collecting the transmittedreflected pulses to obtain collected pulses; detecting the collectedpulses to obtain a plurality of corresponding ultrasonic electricalsignals; and processing the plurality of ultrasonic electrical signalsto obtain a plurality of physical parameter signals which represents thephysical parameter of the at least one layer over the extended area ofthe article.
 39. The method as claimed in claim 38 further comprisingthe steps of moving the article relative to the sensor station andgenerating a position signal representative of the position of thearticle at the sensor station and wherein the physical parameter signalsare processed with the position signal to locate a defect in theextended area of the article.
 40. The method of claim 39 furthercomprising the step of generating a control signal based on location ofthe defect.
 41. The method of claim 40 further comprising the step ofcoating the surface of the article as a function of the control signal.42. A system for measuring a physical parameter of at least one layerover an extended area of a multilayer article without damaging thearticle, the system comprising:a generation laser for generating a firstpulse of electromagnetic energy; a first device for directing the firstpulse into a first plurality of beams; a first plurality of opticalfibers for transmitting the first plurality of beams therethrough to acorresponding plurality of locations at a sensing station; a detectionlaser for generating a second pulse of electromagnetic energy; a seconddevice for directing the second pulse into a second plurality of beams;a second plurality of optical fibers for transmitting the secondplurality of beams therethrough to the corresponding plurality oflocations at the sensing station; a plurality of sensor heads, each ofthe sensor heads positioned at one of the locations at the sensingstation and coupled to one of each of the first and second pluralitiesof optical fibers, each of the sensor heads having at least one opticalcomponent for directing one of the first plurality of beams toward oneof a first plurality of spots on a surface of the article to generate anultrasonic wave in the article which, in turn, causes ultrasonic motionof the surface of the article without damaging the article, each of thesensor heads also having at least one optical component for directingone of the second plurality of beams toward one of a second plurality ofspots on the surface of the article which substantially overlaps itscorresponding spot of the first plurality of spots to obtain a reflectedpulse of electromagnetic energy which is modulated based on theultrasonic motion of the surface at its respective first and secondoverlapping spots, each of the sensor heads also receiving its reflectedpulse; a third device coupled to each of the sensor heads for collectingthe reflected pulses to obtain collected pulses; an optical detectorcoupled to the third device for detecting the collected pulses to obtaina plurality of corresponding ultrasonic electrical signals; and a signalprocessor for processing the plurality of ultrasonic electrical signalsto obtain physical parameter signals which represents the physicalparameter of the at least one layer over the extended area on thearticle.
 43. The system as claimed in claim 42 further comprising amechanism for moving the article relative to the sensor station and asensor coupled to the mechanism and the signal processor to generate aposition signal representative of position of the article relative tothe sensor station and wherein the physical parameter signals areprocessed with the position signal by the signal processor to locate adefect in the article at the extended area of the article.
 44. Thesystem as claimed in claim 43 further comprising a controller coupled tothe signal processor for generating a control signal based on locationof the defect in the article.
 45. The system as claimed in claim 44further comprising a surface coating mechanism for coating the surfaceof the article as a function of the control signal.
 46. The system asclaimed in claim 45 wherein the mechanism includes a robot and thecontroller is a robot controller.
 47. The system as claimed in claim 42wherein the second plurality of optical fibers couple the sensor headsto the third device.
 48. The system as claimed in claim 42 furthercomprising a third plurality of optical fibers for coupling the sensorheads to the third device.